Sh!tty parents make for equally sh!tty grandparents.
That's my take on it.
If they didn't know how to be a parent they will not know how to be a grandparent.
My kids got two great Grannies, unfortunately they live on the other side of the globe.
But I'm replying because I have two very close mates here and they have the exact same problems you're having OP, so I'm familiar with your situation from what they go through day to day and week to week.
Their children's grandparents might as well be on the other side of the globe for all they contribute to their lives.
And when we examine how they treated their own children, it was as you said,
as if they were a burden.
So I just wanted to tell you that you weren't alone in having those feelings as my friends feel the exact same way.
Your children will come to their own decisions and most likely not want to go see their grandparents, they'll pick that vibe up themselves if they haven't already.
Especially your 6yr old.
Being a grandparent is such a privilege.
It doesn't take much to be a good one.
Be present, be interested , be kind.
For the ones I've encountered here through my friends, they have it all backwards, do absolutely nothing and chuck a new toy in there now and then.
I get so mad when I see my friends as upset as you are now because not only are they wasting such a wonderful chance to get to know their grandkids, but they're so unbelievably ungrateful of the opportunity they have that others (far better and more worthy) have been denied.
I don't know where you go from here, but my own advice would be to cut ties.